Fresenius J Anal Chem (2001) 370 : 151–155

© Springer-Verlag 2001

S P E C I A L I S S U E PA P E R

Mirella Buzoianu

Reference materials and certified reference materials for spectrometry in Romania

Received: 23 October 2000 / Revised: 11 December 2000 / Accepted: 16 December 2000

Abstract Several reference materials (RMs) and certified reference materials (CRMs) are widely used in Romania as measurement standards in different spectrochemical measurements. Among them, single element standard solution certified for their mass concentration play a key role in ensuring the required traceability of results expressed in this measurement unit. A short review of the locally available elemental RMs and CRMs used in atomic spectrometry or in other analytical techniques where aqueous standard solutions are required (usually called RMs or CRMs for spectrometry) is given. The experience of the INM in preparation and certification of such materials is described. Some aspects regarding their use for ensuring the accuracy and for confirmation of the traceability of analytical measurements, especially through calibration and metrological validation of main instrument performances, are discussed.

1 Introduction The use of RMs and CRMs is confined to certain measurements performed in Romania and described under the following aspects: (a) types of RMs and CRMs developed for spectrometry, (b) CRMs as base of traceability and (c) role of CRMs in metrology related activities. Spectrochemical measurements are important parts of analytical chemistry and they are applied in many other testing fields and scientific disciplines. Results of spectrochemical measurements have become more and more important for legal decisions in trade, environmental protection, medicine and science on a national and international scale. Therefore, RMs and CRMs are important tools, but not the only, to obtain the necessary comparability and traceability of such results.

M. Buzoianu () National Institute of Metrology, Sos.Vitan-Bârzesti nr.11, 75669 Bucharest, Romania e-mail: [email protected]

For mutual recognition of measurement, the confidence for all RMs and CRMs used both in field measurements and for metrological purposes needs to be established and the traceability should be demonstrated. Within this framework, the paper describes the experience of the INM in the field of elemental RMs and CRMs used in calibrating and testing the instruments used in atomic absorption spectrometry and in other analytical techniques where aqueous standard solutions are required.

2 A brief review of RMs and CRMs related activities in Romania Uniformity of all measurements performed in Romania is the main goal of the legal metrology activities in accordance with the Romanian law of metrology [1] and the regulations issued in the field of metrology. Therefore, measurements in trade, in production and testing of pharmaceuticals or in the field of health, safety and environmental protection are performed in a coherent measurement system within which the consistency of measurements is easily maintained and demonstrated, and the development of proper RMs and CRMs is highly requested. The use of RMs to transfer the values of measured or assigned quantities between testing, analytical and measurement laboratories is increasingly demanded. Also, they are widely used for calibration of measuring equipments and for the evaluation or validation of measurement procedures and instruments. In certain cases, RMs and CRMs enable properties to be expressed in conventional unit. Main fields where RMs and CRMs are used accordingly to their definition [2] are illustrated in Fig. 1a and 1b, respectively. On the other hand, many measurements performed for different testings in other fields than those above mentioned, such as metallurgy, materials science, geology, cement industry, etc., mostly using matrix RMs and CRMs, are subject to quality control and quality assurance procedures demanded by third-party accreditation and certification.

152 Fig. 1 Main fields where RMs and CRMs are used

Therefore, several ISO Guides describing the definitions, certification approaches and the use of RMs were adopted as national standards [3–5] and stand as main references in any RMs and CRMs related activities. In present, there is an increasing number of RM producers and the demonstration of their scientific and technical competence is a basic requirement for ensuring the quality of RMs. Therefore, the INM has the responsibility for the certification of all RMs produced in Romania. In this respect, the INM also issued a schematic diagram for hierarchy of RMs [6], as well as several national Legal Metrology Norms (NML), based on OIML Recommendations, describing principles and main requirements for RMs and CRMs used in metrological activities regarding spectrochemical measurements [7–10]. To ensure the traceability of all chemical measurements, as required by the Romanian Law [1], the INM developed several types of RMs and CRMs for different other physicochemical properties and for chemical composition. Aspects of the national system for RMs in Romania have been described [11].

3 Elemental RMs and CRMs used for spectrometry 3.1 Types of elemental RMs and CRMs for spectrometry issued Calibration of instruments based on atomic spectrometric methods, such as atomic absorption (AA) spectrometers and inductively coupled plasma spectrometers is usually

performed in the field analytical laboratories using aqueous standard solutions obtained after diluting elemental RMs or CRMs of mass concentration. Such materials are provided both by national or international producers. In Romania, mainly, the INM issued up to now 14 types of elemental CRMs for spectrometry, 8 types of RMs were issued by a national producer and other types are in development. A list of the elemental CRMs for spectrometry issued by the INM is given in Table 1.

Table 1 CRMs for spectrometry issued by the INM CRM

Destination

Element

13.06 12.03 13.09 13.12 13.01 13.05 13.13 13.04 13.02 13.07 13.08 12.02 12.01 13.03

Cadmium Calcium These CRMs are intended Chromium as standard stock solution Cobalt of 1.000 g/L for use in Copper atomic absorption spectrometry, Lead ICP spectrometry or any other Lithium analytical technique that Iron requires aqueous solution for Magnesium calibrating, verification and Manganese validation of instrument Nickel metrological performances Potassium Sodium Zinc

Acid concentration HNO3 10% HCl 10% HCl 10% HNO3 10% HNO3 10% HNO3 10% HCl 1% HNO3 10% HNO3 10% HNO3 10% HNO3 10% HCl 1% HCl 1% HNO3 10%

153 Table 2 Preparation steps and mathematical relationship considered for the evaluation of mass concentration and uncertainty Preparation steps

Main input quantities and environment conditions

Calculate the necessary amount of elemental form or chemical substance, msubst

Atomic weights; Purity of the material

msubst =

Recalculate mass relative to balance indication, mdispl

Density of material; Density of weight; Air density

ρ   ρ  Balance sensitivity  mdispl = msubst ⋅  1 – air  /  1 – air   ρ subst   ρw 

Weighting the recalculated mass of initial substance, m

Temperature; Calibration of the balance ρ ρ Atmospheric pressure; m = mdispl ,real ⋅  1 – air  /  1 – air  2 2  ρw   ρ subst  um = um , A + um , B Mass indication

Dissolution and filling to volume V

Final volume; Temperature

Calculation of mass concentration, c

c=

Preliminary verification

m ⋅ xL ⋅ y V

Mathematical relationship

Main sources of uncertainty

Mass fraction of substance in the initial substance

c0 ⋅ V ML xL = y ⋅ xL M subst

2

 ML   uM L  ux L =   + 2   M subst   M subst 

2

Calibration of the flasks  ∂Vglass   ∂Vsol  ⋅ u 2 uV = uV , A 2 + uV , flask 2 +   ⋅ ut2 +  t  ∂t  ∂t  2

2

2

c 2 c 2  c  c  uc =   ⋅ um2 +   ⋅ u x2 L +   ⋅ u y2 +   ⋅ uV2  xL   y m V

Volumetric methods; Molar spectrophotometry

RMs used to calibrate the spectrophotometer; CRMs for volume meas.

msubst is the mass of initial substance, g; mdispl – the mass indicated by the balance, g; mdispl,real – the real mass of initial substance indicated by the balance, g (depending on the display of the balance and on the form in which the initial substance is delivered); m – real determined mass of initial substance, g; V – final volume, L; Vflask – flask volume, L; c0 – given concentration, g/L; c – assigned concentration of element, g/L; y – mass fraction of substance in the initial substance, 1;

xL – mass fraction, atomic mass of the element L to the molecular mass of the initial substance, 1; ρair – air density, kg/m3; ρw – density of weight, kg/m3; ρsubst – density of the substance, kg/m3; ML – atomic weight of the element, kg/mol; Msubst – atomic weight of the element, kg/mol T – temperature, °C; u – standard uncertainty; subscript A denotes type A uncertainty; subscript B denotes type B uncertainty

These materials are gravimetrically prepared from high purity metals and salts at a nominal value of 1.000 g/L using traceable calibrated balance and glassware (Class A). As a common practice, the initial purity of metals or salts used is checked by spectrographic or ICP methods. Then, the final value of mass concentration is also verified by an independent method (titrimetry for instance). Beside the single elemental CRMs for spectrometry described in Table 1, the INM developed some multi-element CRMs used in environment protection applications.

dent method used to verify the mass concentration value after preparation. Accordingly, Table 2 describes main preparation steps and mathematical relationships considered for the evaluation of mass concentration and uncertainty. This approach is in accordance with the requirements given in [4] and the mathematical algorithm follows the recommendations given in [12]. Working under the described condition, the expanded uncertainty of the certified value of mass concentration does not exceed 0.002 mg/L (for Cd, Cu, Cr, Co, Pb, Mg, Mn, Ni and Zn) and 0.003 mg/L (for Ca, Li, Fe, K and Na). Information provided by the Calibration Certificate of the CRM, issued by the INM, include: material batch, certified property – mass concentration, main characteristics (number of vials, vial type, volume, elemental composition, etc.), higher standards used for calibration, environment temperature, humidity, specific instrumental conditions, calibration method used and the traceability of the CRMs. The second approach is based on the ANOVA method of treatment of measurement results obtained in the interlaboratory tests. Mostly, elemental RMs produced by Ro-

3.2 Experience of the INM in certification of RMs for spectrometry Two approaches are used to certify a value of mass concentration. The first one relies on a detailed evaluation of the preparation uncertainty (which considers all sources of uncertainty corresponding to the steps followed in the preparation process) and on the uncertainty of the valid indepen-

154 Fig. 2 Attempt to demonstrate the traceability of mass concentration measurement results

manian manufacturers of such materials are subject to this approach. Firstly, the manufacturer of the material has to demonstrate the validity of the procedure of preparation and the homogeneity of the material. Then, several (10 at least) independent, equally competent laboratories measure the mass concentration of samples of material. The results reported by these laboratories are tested for their compliance with the measurement method accuracy and repeatability and are subject to tests for statistical significance. Finally, both the consensus value and the variance of consensus value are reported. This approach was applied to some elemental RMs for spectrometry produced in national area. A maximum experimental amplitude of 0.004 mg/L was obtained. An expanded uncertainty of 0.002 mg/L was certified for a mass concentration of 1.000 g/L for the following elements: Cu, Fe, Pb, Zn, Mg and Ca.

3.3 On the use of RMs and CRMs in metrology related activities Elemental CRMs for spectrometry issued by the INM are mainly used in the following fields: • Method development and evaluation – Verification of precision and accuracy of instruments – Evaluation of field methods

– Validation of instrument performances for a specific use • Establishment of measurement traceability – Development of traceability protocols – Direct field use • Assurance of measurement of compatibility – Direct calibration of instruments – Intra- and inter-laboratory quality assurance Mostly, periodic verification of precision and accuracy of instruments, validation of instrument performances for a specific use within pattern approval tests, direct calibration of instruments and development and demonstration of traceability protocols are the legal metrological activities which requires the use of elemental CRMs for spectrometry. Examples of the use of RMs and CRMs in different application for calibration of atomic absorption spectrometers, for the evaluation of measurement uncertainty as well as for testing the instrument performances were described in [13–15]. In Fig. 2 a general attempt to demonstrate the traceability of mass concentration measurement results is given. Figure 3 illustrates how a CRM issued by the INM is used during the metrological verification of atomic absorption spectrometers. A sample of 2.000 ± 0.008 mg/L, prepared by diluting the CRM type 13.01 (Cu) of 1.001 ± 0.002 g/L, was measured against several types of spectrometers (Zeiss

155 Fig. 3 Some results on the use of CRMs to verify instrument performances of AA spectrometers for measuring metal pollutants in water

Jena, Perkin Elmer, ATI Unicam and IAUC type). Each instrument was calibrated against minimum of three standard solutions prepared either from certified standard solutions of 1.000 g/L supplied from various manufacturers or from pure substances. Taking into account the expanded measurement uncertainty (k = 2) each measurement result was reported as a confidence range. The upper ((a) series) and the lower ((b) series) confidence limits for each measured mass concentration are illustrated as small quadrates, joined by a dotted line. Also, the characteristic concentration (sensitivity) of each instrument tested are illustrated in Fig. 3 as small triangles. The deviation of the mean value measured on each instrument from the value of the sample laid within a range of (–18.4...13.5) % (rel). Even instruments of low sensitivity produced measurement results within the confidence limits of the standard solution used. Poor measurement accuracy did not result in a large measurement uncertainty.

4 Conclusions The paper describes several aspects regarding preparation and certification of elemental RMs and CRMs for spectrometry issued in Romania, mainly within the INM. The following conclusions can be drawn: • INM attempts to develop a set of CRMs (single and multi-element solution form) for spectrometry necessary to assure, from the metrology point of view, the mass concentration within the analytical field laboratories. • Mainly the CRMs for spectrometry are prepared at a nominal concentration of 1.000 g/L with an uncertainty within (0.002 to 0.003) g/L, following a valid gravimetric method.

• CRMs for spectrometry issued by the INM are widely used for calibration, validation of instrument performance and to develop traceability protocols. • Simulating real condition of measurement, to a certain extent, the CRMs developed for spectrometry were useful to evaluate measurement uncertainty of a real life measuring system.

References 1. Romanian Law of Metrology, issued in 1992 and amended in 1999 2. International Vocabulary of basic and general terms in metrology (1993) International Organisation of Standardization (ISO) 3. SR 13250–1:1994 “Reference materials. Terms and definitions” (corresponding to ISO Guide 30:1992) 4. SR 13250–2:1995 “Reference materials. Guide to general and statistical principles for the certification of reference materials” (corresponding to ISO Guide 35:1989) 5. SR 13250–3:1995 “Reference materials. Guide to the content of certificates of reference materials” (corresponding to ISO Guide 31:1981) 6. Schematic Diagram for the hierarchy of reference materials, 1999 7. NML 9–02–94 ‘Atomic absorption spectrometers used for measuring metal pollutants in water’ (in accordance with OIML R 100) 8. NML 9–12–97 ‘UV-Vis molecular spectrophotometers’ 9. NML 9–04–94 ‘Spectrophotometers for medical use’ 10. NML 9–11–97 ‘Photometers for medical use’ 11. Buzoianu M, Duta S (1996) Proceedings of CERM’96, pp 27– 31 12. EURACHEM/CITAC Guide (2000) ‘Quantifying Uncertainty in Analytical Measurement’, 2nd edition 13. Buzoianu M (1998) Accred Qual Assur 3:328–334 14. Buzoianu M (2000) Bull. OIML XLI, No.1, 5–12 15. Buzoianu M (2000) Accred Qual Assur 5:231–237